Use a Pregnancy Calculator to estimate your due date and how many weeks pregnant you are right now. You enter a start date (usually your last menstrual period) and the calculator computes gestational age, due date, and remaining weeks.
These estimates help with planning appointments and tracking milestones, but they are not a medical diagnosis. Your clinician confirms dates using ultrasound and prenatal exams.
What a Pregnancy Calculator actually calculates
A Pregnancy Calculator estimates pregnancy timing using a standard medical convention: gestational age is counted from the first day of your last menstrual period (LMP). That’s why dates can differ from the day of conception.
Most calculators provide three core outputs:
- Gestational age (weeks + days): how far along you are.
- Estimated due date (EDD): the expected delivery date based on 40 weeks from LMP.
- Remaining time: weeks and days left until the due date.
Key dates and terms (simple definitions)
Last menstrual period (LMP)
Your LMP is the first day of your most recent period before pregnancy. Many pregnancy dating methods use this date as the starting point.
Conception date
Conception happens when an egg is fertilized. Because ovulation usually occurs about 2 weeks after LMP, conception-based estimates typically require converting to gestational age.
Gestational age
Gestational age is the medical way clinicians track pregnancy progress. It counts from LMP and is expressed as weeks and days.
The formulas behind the calculator
Pregnancy calculators use date arithmetic plus a standard conversion between weeks and days.
1) Estimate due date (EDD) from LMP
Assuming a typical 40-week gestation counted from LMP, the estimated due date is:
| Variable | Meaning |
|---|---|
| EDD | Estimated due date |
| LMP | Last menstrual period start date |
| 40 weeks | Standard gestational length |
EDD = LMP + 280 days (because 40 weeks × 7 days/week = 280 days).
2) Gestational age from LMP
To compute how far along you are on a chosen “today” date:
Gestational days = Today − LMP
Then convert days to weeks and days:
- Weeks = floor(gestational days ÷ 7)
- Days = gestational days mod 7
3) Remaining time until the due date
Remaining time is calculated as the difference between your due date and today:
Remaining days = EDD − Today
- Remaining weeks = floor(remaining days ÷ 7)
- Remaining days = remaining days mod 7
4) If you enter a conception date instead of LMP
Many people know the approximate conception window. A common dating conversion is that LMP is about 14 days (2 weeks) before conception.
LMP ≈ Conception − 14 days
The calculator then uses the same LMP-based formulas above to estimate gestational age and due date.
How to use the Pregnancy Calculator (step-by-step)
- Choose your input type: LMP-based or conception-based.
- Enter the start date using the date picker.
- Set “today”: use the default or pick a different date for planning.
- Click Calculate to see gestational age, due date, and remaining time.
If you enter a future date or an invalid date, the calculator will prompt you to correct the input.
Practical example 1: Using your LMP
Suppose your LMP was January 10 and today is March 5.
- EDD estimate: January 10 + 280 days → around October 17.
- Gestational age on March 5: counted in weeks and days from January 10.
- Remaining time: the difference between today and the estimated due date.
This helps you estimate which trimester you’re in and when you may schedule common prenatal milestones.
Practical example 2: Using an approximate conception date
Suppose you believe conception happened around May 1 and today is June 20.
- Converted LMP estimate: May 1 − 14 days → around April 17.
- EDD estimate: April 17 + 280 days → around January 21.
- Gestational age: computed from the converted LMP to today.
Conception-based estimates can be useful, especially if you tracked ovulation, but ultrasound dating may adjust the final due date.
What can change your due date (and why)
Due dates are estimates. Clinicians often adjust them after an ultrasound, especially early in pregnancy, when measurements are most accurate.
- Ultrasound measurements can shift gestational dating.
- Cycle length varies; some people ovulate earlier or later.
- Irregular periods can make LMP less reliable.
- Fertility treatments may provide more precise timing.
Bottom line: use the Pregnancy Calculator for planning, then rely on your clinician’s dating confirmation for medical decisions.
Frequently Asked Questions
How accurate is a Pregnancy Calculator for due dates?
A Pregnancy Calculator provides an estimate based on standard gestational timing from LMP (40 weeks). Accuracy varies because ovulation and cycle length differ. Clinicians confirm dating with ultrasound, especially in the first trimester. So treat results as planning estimates, not a medical guarantee.
If I know my conception date, can I use it in a Pregnancy Calculator?
Yes. Many calculators convert conception to an estimated LMP by subtracting about 14 days, then compute gestational age and due date. This works best when conception timing is fairly accurate, such as with ovulation tracking. Ultrasound can still adjust the final due date.
What if my due date is different from my doctor’s?
It’s common for due dates to change after ultrasound dating. Your doctor may use measurements that better reflect your pregnancy’s true timing. If your LMP is uncertain or your cycles are irregular, a calculator may be off. Ask which date your clinician considers official.
Why does gestational age not equal “time since conception”?
Gestational age counts from the first day of the last menstrual period, not from conception. Conception usually happens about two weeks after LMP. That’s why gestational weeks can be ahead of how long you think you’ve been pregnant. This method is standard for prenatal care.
Can I use the Pregnancy Calculator for multiple pregnancies?
Yes for timing estimates, but medical care differs for twins or higher-order multiples. Due dates can still be estimated using gestational age from LMP or conception, but clinicians may monitor growth and development more closely. Always follow your obstetric team’s guidance for appointments and delivery timing.
When to talk to a clinician
Contact your healthcare provider if you have severe pain, heavy bleeding, or concerns about your pregnancy dating. They can review your history and use ultrasound to confirm gestational age.
You can also use this Pregnancy Calculator to prepare questions for your first prenatal visit, such as what due date your clinician will use and why.